Gas discharge lamps such as fluorescent lamps produce light by exciting a gas with a high voltage a.c. signal generated by the drive section of a ballast circuit. The intensity of illumination is determined by the power of the excitation signal, which in turn depends on the signal frequency.
A ballast circuit for a gas discharge lamp is illustrated in U.S. Pat. No. 5,965,985, which issued to Nerone and is commonly assigned to the owner of the present application. U.S. Pat. No. 5,965,985 is incorporated herein by reference in its entirety. FIG. 1 shows Nerone's ballast circuit 10. A gas discharge lamp 12 is powered from a d.c. bus voltage between d.c. bus 16 and reference bus 18 that is converted to a.c. Switches 20 and 22 are serially connected between buses 16 and 18 in the form of a complementary converter circuit. The switches comprise n-channel and p-channel enhancement mode MOSFETs connected in a common source arrangement at a common node 24. The switches may alternatively comprise other devices having complementary conduction modes, such as PNP and NPN bipolar junction transistors.
A resonant load circuit 25 including a resonant inductor 26a and a resonant capacitor 28 receives current through the common node 24. Circuit 25 includes a d.c. blocking capacitor 30 and a snubber capacitor 32. Lamp 12 includes resistively heated cathodes 12a and 12b that are supplied with heating current by windings 26c and 26d mutually coupled to inductor 26a. Switches 20 and 22 cooperate to provide a.c. current from the common node 24 to resonant inductor 26a.
The gate electrodes 20a and 22a of the switches are connected at a control node 34. Gate drive circuitry, generally designated 36, is connected between control node 34 and common node 24, and implements regenerative control of switches 20 and 22. A drive winding 26b is mutually coupled to resonant inductor 26a, which carries a voltage proportional to the instantaneous rate of change of current in load circuit 25. A transformer winding 38a serially connected to the driving inductor 26b couples a controlled voltage in series with the driving inductor 26b as described below.
A bidirectional voltage clamp 40 comprised of back-to-back Zener diodes cooperates with the transformer winding 38a such that the phase angle between the fundamental frequency component of voltage across resonant load circuit 25 (e.g., from node 24 to node 18) and the a.c. current in resonant inductor 26a approaches zero during lamp ignition. A capacitor 44 is provided between nodes 24 and 34 to predictably limit the rate of change of control voltage between the nodes. This provides a dead time interval during switching of switches 20 and 22 during which neither switch is turned on.
The frequency of the a.c. signal produced by the ballast is controlled by a clamping circuit 62. FIG. 2 shows details of Nerone's clamping circuit. The clamping circuit controls ballast circuit frequency by varying the loading across the transformer winding 38a by means of a controlled impedance, implemented as a MOSFET 72, in response to an error signal. The error signal is produced by a difference amplifier 64 that receives as input a set point signal provided by a user input potentiometer 68, and a lamp current feedback signal provided by low pass filter 60 of FIG. 1. The low pass filter 60 provides a time-averaged signal derived from a lamp current signal sensed by a sensing resistor 54 and rectified by p-n diode 56. Half cycles of lamp current of the other polarity are shunted across resistor 54 by a diode 58. The error signal provided by the difference amplifier 64 is amplified by an error amplifier 70, powered from a node 73, and applied to the gate of the MOSFET 72. The MOSFET 72 determines the voltage across a control winding 38b, which is mutually coupled to the transformer winding 38a of the driving circuit of FIG. 1.
A diode bridge network 74a-74b enables the MOSFET 72 to conduct current through winding 38b in both directions, e.g., first through diodes 74a, 74b and then through diodes 75a, 75b. A capacitor 78 shunts MOSFET 72 to assist in clamping voltage across the control winding. A voltage clamp 80 such as a Zener diode shunts MOSFET 72 to limit the minimum frequency so as to set a maximum voltage across the lamp during ignition. The lower node of MOSFET 72 comprises the reference bus 18 of FIG. 1, and upper node 73 comprises a power supply node coupled via a resistor (not shown) to the d.c. bus 16 of FIG. 1.
A preheat switch 82, such as a p-channel enhancement mode MOSFET, may be provided to conduct for a preheat timing interval when the ballast circuit is first supplied with d.c. bus voltage. When conducting, switch 82 overrides MOSFET 72 by shorting its output. This allows resistively heated cathodes 12a and 12b of FIG. 1 to reach a desired temperature, while maintaining a low voltage across lamp 12, before lamp ignition. A circuit 84 for controlling the preheat switch 82 may be constructed as shown in FIG. 3. As shown in FIG. 3, a comparator 85 receives a reference voltage from circuit 86 on its negative input, and upon bus energization, receives an increasing voltage on its positive input from a preheat capacitor 88. The capacitor is charged by current conducted from node 73 by a preheat resistor 90. The values of resistor 90 and capacitor 88 determine the duration of the preheat period during which switch 82 of FIG. 2 conducts upon bus energization.
Nerone's ballast circuit thus enables control of the brightness of a fluorescent discharge lamp by means of the user input, which controls the voltage across the clamping transformer winding 38a of the driving circuit, thereby determining the frequency of the a.c. voltage applied to the resonant load circuit and thus controlling the current provided to the gas discharge lamp.